Method and apparatus for locating a wireless user

ABSTRACT

A plurality of antennas transmit a first spread spectrum signal having an associated code. The first spread spectrum signal is received at the wireless user. For each received first spread spectrum signal, a second spread spectrum signal is transmitted having an associated code having a same phase as that received first spread spectrum signal. The second spread spectrum signals are received at the plurality of antennas. A distance measurement is determined between each antenna and the wireless user based on in part a received timing of the second signals. The wireless user&#39;s location is determined based on in part the distance determinations.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/663,240, filed Sep. 16, 2003, which is a continuation of U.S. patentapplication Ser. No. 10/096,015; filed Mar. 12, 2002; which is acontinuation of U.S. patent application Ser. No. 08/539,276; filed Oct.4, 1995, which issued on Apr. 2, 2002 as U.S. Pat. No. 6,366,568; whichis a divisional of U.S. patent application Ser. No. 08/301,230; filedSep. 6, 1994, which issued on Mar. 25, 1997 as U.S. Pat. No. 5,614,914,which applications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to two way wireless communication systems.In particular, the present invention relates to wireless telephonesystems with diversity signal transmission for reducing signal fadingand measuring subscriber location.

BACKGROUND OF THE INVENTION

Wireless radio communication is subject to the adverse effects of signalfading, in which the signal level at the receiver temporarily losesstrength for a variety of reasons, such as from variable multipathreflections causing signal cancellation, time varying transmission lossdue to atmospheric conditions, and mobile receiver movement introducingobstructions into the signal path, and the like. Signal fading causespoor reception, inconvenience, or in extreme cases, a loss of callconnection.

It is known to use various forms of signal diversity to reduce fading.For example, as indicated in U.S. Pat. No. 5,280,472, signal diversitymitigates the deleterious effects of fading. There are three major typesof diversity: time diversity, frequency diversity and space diversity.

Time diversity is obtained by the use of repetition, interleaving orerror correction coding, which is a form of repetition. Error detectiontechniques in combination with automatic retransmission, provide a formof time diversity.

In frequency diversity, signal energy is spread over a wide bandwidth tocombat fading. Frequency modulation (FM) is a form of frequencydiversity. Another form of frequency diversity is code division multipleaccess (CDMA) also known as spread spectrum. Due to its inherent natureas a wideband signal, CDMA is less susceptible to fading as compared toa narrow band modulation signal. Since fading generally occurs in only aportion of the radio spectrum at any one given time, a spread spectrumsignal is inherently resistant to the adverse effects of fading.

Space diversity is provided by transmitting or receiving the same signalon more than one geographically separated antenna. Space diversityprovides alternate signal paths to guard against any one path beingsubject to fading at any one time. Space diversity also creates sometime diversity since the receiver receives the same signal separated bysmall propagation delays. The difference in propagation delay requiresthat the receiver be able to discriminate between the arriving signals.One solution is to use multiple receivers, one for each arriving signal.For instance, it is known from U.S. Pat. No. 5,280,472 to deliberatelyintroduce relatively small delays compared to an information symbol,into a space diversity multiple antenna CDMA system in order to createartificial multipath time diversity signals greater than one chip delayup to a few chips. CDMA systems are capable of discriminating betweenidentical plural signals arriving at the receiver with differentpropagation delays greater than one chip delay. Such receivers are knownas Rake receivers. However, prior art systems require multiple CDMAreceivers, one CDMA receiver for each separate received CDMA signal. Itis desirable to provide a system for receiving time diversity CDMAsignals which does not require multiple CDMA receivers.

Measuring or determining the location of mobile units is well known. Insome systems, fixed antennas measure the mobile location. In othersystems, the mobile unit determines its location from multiple receivedsignals. If the system is two way, the communication link permits boththe mobile subscriber and the fixed system to exchange location data.Various known systems use satellites or multiple antennas to provideinformation on the location of a mobile subscriber. For example,multiple directional receiving antennas can be used to triangulate theposition of a mobile transmitter. In such systems, the stationaryreceivers determine the mobile subscriber location; in other systems,the mobile subscriber determines its location from the received signals.For example, the Global Position System (GPS) is a multiple satellitesystem providing signals which permit a mobile subscriber station todetermine its position in latitude and longitude. However, bothsatellite systems and the GPS receivers for receiving satellite signalstend to be expensive.

The combination of a GPS receiver and a cellular telephone is shown inU.S. Pat. No. 5,223,844. Such combination provides useful services, asfor example a security alarm service to deter car theft, in whichtripping the alarm also alerts the security service to the location ofthe car. Generally, it is desirable to provide a system which combinestelephone or data service with location measurement at a reasonablecost.

It is desirable to provide a system of time diversity signals using timedivision multiple access (TDMA) in various combinations with CDMA andspace diversity antennas, to provide a variety of systems which resistfading, reduce receiver cost, and provide location measurement formobile subscribers.

SUMMARY OF THE INVENTION

The present invention is embodied in a wireless communication system inwhich transmission diversity is used to reduce fading and simplifyreceiver design. The present invention is further embodied in a wirelesscommunication system in which time division signals are code division(spread spectrum) multiplexed onto space diverse antennas to provide awireless communication system with the ability to determine subscriberlocation using the same communication signals which are used for theprimary wireless communication.

A cellular system utilizes transmission diversity by transmitting aplurality time slotted signals for communicating data. A transmittingstation including a transmitter configured to transmit a plurality ofdata signals corresponding to the data in different time slots. The timeslot occurring at different times so that transmitted data signals arenot simultaneously transmitted. A receiving station includes a receiverfor sequentially receiving the data signals and a combiner forreconstructing the data from the received data signals. Preferably, thetransmitting station has a plurality of spaced apart antennas from whichthe respective data signals are transmitted. Preferably, the datasignals are transmitted from the transmitting station as a plurality ofPN spread spectrum signals which are formed by spreading and modulatingthe data with different PN codes. The receiving station thenreconstructs the data by first despreading and demodulating the receiveddata signals. The receiving station may be configured to measure therespective time of arrival of the transmitted data signals and computeits location from said respective measured time of arrival of the datasignals. The transmitting station may be a combined base station andtransfer station.

Specifically, a data packet which, for example may carry telephone voicetraffic, is transmitted at three different times from three differentantennas. The receiver thus receives the same data packet at threedifferent times from three different antennas. The receiver uses thebest data packet or combination of the data packets to reduce theeffects of fading.

In addition, the receiver uses the absolute and extrapolated relativetime of arrival of the three data packets to determine its location fromthe three transmitting antennas. First, absolute range to one antenna isdetermined by the time required for a round trip message. Then, therelative time of arrival of data packets, referenced to a universaltime, from the two other antennas indicates the relative distances ascompared to the first antenna. Since all three transmitting antennas areat known fixed locations, the receiver computes its own location as theintersection of three constant distance curves (in the two dimensionalcase, circles, or in the three dimensional case, the intersection ofthree spheres). In the alternative, the mobile subscriber stationprovides raw delay measurement data back to a fixed station, or locationservice center, which computes the mobile subscriber location.

More particularly, the present invention is embodied in a system usingCDMA to modulate a TDMA signal which is transmitted from three spacediversity antennas. In a first embodiment, the TDMA signals are used totransmit multiple repetitions of the same data packet from a transferstation with three space diversity antennas. In a second embodiment, theTDMA signals are used to transmit multiple repetitions of the same datapacket from three transfer stations each transfer station including oneof the three space diversity antennas. The data packets could either beidentical, or could carry substantially the same information, butmodulated with different spreading codes or different segments of thesame spreading code.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a system diagram of a wireless telephone distribution systemincluding a first embodiment of a transfer station in accordance withthe present invention.

FIG. 2 is a block diagram of a first embodiment of a wireless telephonedistribution system in accordance with the present invention.

FIG. 3 is a system diagram of a first embodiment of a wireless telephonedistribution system in accordance with the present invention.

FIG. 4 is a system diagram of a wireless telephone distribution systemincluding a second embodiment of a transfer station in accordance withthe present invention.

FIG. 5 is a system diagram of a second embodiment of a wirelesstelephone distribution system in accordance with the present invention.

FIG. 6 is a block diagram of a second embodiment of a wireless telephonedistribution system in accordance with the present invention.

FIG. 7 is timing diagram of a time division multiplex signal whichmodulates a code division multiplex signal in accordance with thepresent invention.

FIGS. 8 and 9 are block diagrams of a first embodiment of a transferstation in accordance with the present invention.

FIG. 10A is a time slot assignment diagram of a wireless telephonedistribution system in accordance with the present inventionillustrating the time division multiplexing and code divisionmultiplexing for 6 simultaneous calls.

FIG. 10B is a time slot assignment diagram of a wireless telephonedistribution system in accordance with the present inventionillustrating the time division multiplexing and code divisionmultiplexing for 12 simultaneous calls.

FIGS. 11A and 11B are a time slot assignment diagram of a wirelesstelephone distribution system in accordance with the present inventionillustrating the time division multiplexing and code divisionmultiplexing for 24 simultaneous calls.

FIG. 12 is a block diagram of a second embodiment of a transfer stationin accordance with the present invention.

FIG. 13 is a block diagram of a subscriber station in accordance withthe present invention.

FIG. 14 is a block diagram of a centralized and integrated transferstation in accordance with the present invention.

FIG. 15 is a block diagram of a transfer station antenna implementation.

FIG. 16 is a block diagram of a distributed antenna implementation ofthe present invention using coaxial cable or fiber optic cable.

FIG. 17 is a timing diagram of a time division multiplex signal which isa modulated code division multiplex signal in accordance with thepresent invention.

FIG. 18 is a system diagram illustrating a distributed antennaimplementation of the present invention.

FIG. 19 is a block diagram illustrating a system in accordance with thepresent invention wherein the location center is external to thecommunication system.

FIG. 20 is an illustration of a system in accordance with the presentinvention for determining location of a mobile subscriber station.

FIG. 21 is a system diagram in accordance with the present inventionillustrating a method for determining location of a mobile subscriberstation.

FIG. 22 is a timing diagram illustrating a method for determining thedistance from a subscriber station and to a transmitting transferstation.

FIG. 23 is a timing diagram illustrating a method for determining therelative distances from a subscriber station to two transmittingtransfer stations.

DETAILED DESCRIPTION System Description First Embodiment FIGS. 1, 2, 3,8, 9

In a first embodiment of the invention shown in FIG. 1, a mobile userhaving an antenna 10 is coupled to a CDMA transfer station 14. The CDMAtransfer station 14 further includes an antenna T, 16, antenna A, 11,antenna B, 12, and antenna C, 13. Antennas A, B and C can be mountedeither on separate structures as is shown, or on a single mast. The onlyphysical requirement is that the space between antennas should besufficient for uncorrelated space diversity. While a quarter wavelengthspacing may be sufficient, at least ten wavelengths is preferable. At 1GHz, 10 wavelengths is about 30 feet, while at 5 GHz, 10 wavelengths isabout 6 feet.

The mobile subscriber antenna 10 (also referred herein as the userterminal antenna, or the subscriber station antenna, or simply antennaU) is coupled by a bidirectional radio link to antennas A, B and C. TheCDMA transfer station 14 is further coupled by a bidirectional radiolink through antenna T through appropriate switching to the publicswitch telephone network.

In operation, forward channel telephone voice traffic received in datapackets on antenna T is transmitted on antenna A during time slot 1,repeated on antenna B during time slot 2, and further repeated onantenna C during time slot 3. All three repeated data packets aresequentially received on antenna 10. In the reverse direction, datapackets representing telephone voice traffic transmitted from antenna 10are substantially simultaneously received on antennas A, B and C. TheCDMA transfer station 14 further retransmits data packets received inthe reverse direction through antenna T back to the telephone network.

FIG. 2 is an overview diagram of a system in accordance with the presentinvention that includes the different interconnections between thesupporting network, i.e., between the public switched network 20 andswitching center and central processor 22, and the CDMA transferstations 26, 28, 30, 32, 34, 36 and 38.

The user at CDMA subscriber station 42 is linked by antenna 10 to theCDMA transfer station 38 through antennas A, B and C. Antenna T, 39 onCDMA transfer station 38 carries wireless TDMA telephone voice trafficto antenna 25 on base station 24. Each of the other CDMA transferstations are coupled to the switching center 22 by a variety ofinterconnection means. Connection means W between TDMA base station 24and CDMA transfer station 36 is a wireless means, having a TDMA channelstructure with six TDMA slots. The wireless TDMA distributioninterconnection WE may be a commercially available wireless local loopsystem such as the Ultraphone® digital radio telephone system providedby InterDigital Communications Corporation. The TDMA time slot structureis carried through the transfer station to become the time slotstructure for the slotted CDMA signal on the output. Connection means WEis the same as connection W except there are four W modules operating inparallel to provide a basic connectivity for 24 voice channels.Connection means F uses a fiber optic cable that connects between theswitching center 22 to the CDMA transfer station 32 without goingthrough a wireless base station. Since connection means F (fiber opticcable) incorporates a modem with a TDM/TDMA channel structure similar toW and WE it readily interfaces with the transfer station. Connection FT(fiber optic cable carrying standard T1 multiplex) between switchingcenter 22 and CDMA transfer station 30 is a fiber optic cable that usesa standard T1 multiplexer as the channel combining means. Therefore, thetransfer station that handles the WE connection means could readily beadapted to operate with the FT connection means. Connections C (coaxialcable) to CDMA transfer station 26, and CT to CDMA transfer station 28,(coaxial cable carrying T1 standard multiplex) are cable means thatfunction like F and FT respectively. Connection means L to CDMA transferstation 36 is a conditioned line that carries up to a 100 kb/s datastream that has the same structure as the wireless TDMA, connectionmeans W. Connection means LE (not shown) utilizes 4 conditioned lines tofunction in the same way as connection means WE. Connection means PG toCDMA transfer station 34 is a pair gain capability that is interfacedinto a transfer station.

Using a combination of over the air and fiber optic/cable media, toconnect to the transfer stations, and a common output air interface,between the transfer stations and the CDMA user terminals, results in aflexible rapid response and economical solution. In addition, normaltelephone lines conditioned to handle 64 kb/s to 100 kb/s could also beused to replace the TDMA wireless input to the transfer station. It alsois very cost effective to connect the input side of the transfer stationto the output of a pair gain module. Since the air interface remains thesame for all these interconnection means, this extended concept becomesa very cost effective solution and transition vehicle.

In the system diagram of FIG. 3, telephone voice traffic through thepublic switched network 20, is coupled to a TDMA base station 24 havingantenna 25 for the transmission and reception of TDMA signals. Aplurality of CDMA transfer stations 44, 46, 48, 50 and 52 providewireless telephone service for a plurality of subscribers 45 and 47.Each CDMA transfer station includes an antenna T for receiving andtransmitting TDMA signals, as well as separate antenna A, antenna B andantenna C for communicating with mobile subscribers 45 and 47. By way ofexample, the TDMA base station 24 may have a range of 35 mile radiuscovering numerous CDMA transfer stations. Each CDMA transfer station maytypically have a range of five miles and be spaced three miles apart toprovide cellular coverage for the entire area. Subscriber 45 will beserved by CDMA transfer station 46, while subscriber 47 will be servedby CDMA transfer station 50. As subscribers move about the system, adifferent CDMA transfer station will be assigned to serve thatsubscriber.

An alternate embodiment capitalizes on the rich connectivity describedabove to more widely distribute the three antennas that are used to givetransmission space diversity. The wider distribution allows compensationfor not only multipath fading, but fading due to blockage. For instanceif the CDMA user (antenna 10 in FIG. 1) goes behind a building or hillthe signal from all three space diversity antennas, on a single transferstation, would fade.

However, if the energy in each time slot was transmitted from differenttransfer stations as in FIG. 4, there is a high probability the userterminal would not be blocked from all three transfer stations at thesame time. Therefore, it is possible to randomize the effects of fadingdue to blockage and be more similar to multipath fading. Randomizationis accomplished by having the central controller assign the differenttime slots on an individual basis during the call setup process. Whenimplemented using a W or WE connection means, there is little impact onthe capacity between the base stations and the transfer stations, but itwould increase the number of TDMA receivers. However, there is also adiversity improvement on the base station to transfer station link.Generally speaking, the impact on the other hard wired connection meansis even less. A major advantage of using multiple transfer stations astransmission diversity sources is that it allows the user CDMA receiverto evaluate the quality of the signal from each transfer station andrequest a handoff for individual time slots as better links are found,providing a highly reliable and smooth transition as a user passesthrough an area.

System Description Second Embodiment FIGS. 4, 5, 6, 12

FIG. 4 illustrates a wireless telephone distribution system withenhanced space diversity. As before, a mobile user antenna 10 is coupledto antenna A during time slot 1, antenna B during time slot 2 andantenna C during time slot 3. However, each of antennas A, B and C aremounted on separate respective CDMA transfer stations 54, 56 and 58. Inparticular, an antenna A, 60 is provided on CDMA transfer station 54,antenna B, 68 is provided on CDMA transfer station 56, and antenna C, 64is provided on CDMA transfer station 58. Each of the respective transferstations 54, 56 and 58 are coupled through respective antennas 62, 70and 66 to the TDMA wireless digital telephone system. The signalsreceived from antennas A, B and C by the subscriber station antenna 10are similar to that received in the configuration of FIG. 4. However,due to the separation of antennas A, B and C, at separate CDMA transferstations 54, 56, 58, signal diversity both transmitting and receiving,is vastly improved.

The system configuration of FIG. 6 is similar to that of FIG. 2 with theexception that each CDMA transfer station has either an antenna A, orantenna B or an antenna C. For example, CDMA transfer station A, 108,has a separate antenna A, 109. The CDMA transfer station 106 has anantenna B, 107. Similarly, CDMA transfer station 104 has an antenna C,105. Thus, the antenna 10 of CDMA subscriber station 112 receivessignals from each of CDMA transfer stations 108, 106 and 104. Thereceived signals are time division multiplexed in the sense that onlyone of antenna A, B or C is transmitting to antenna 10 at any one time.During transmission, however, antennas A, B and C provide multiple codedivision multiplexed signals to other users.

In this embodiment, each transfer station has only one type of antenna:either antenna A, antenna B or antenna C. A system arrangement coveringa service area is illustrated in FIG. 5. As before, the public switchnetwork 72 is coupled to a TDMA base station 74 having a transmittingantenna 75 covering an area of approximately a 35 mile radius.Throughout the service area, CDMA transfer stations are spaced apart inone direction 84, and in another direction 86 are positioned to coverthe service area. For illustration, a regular placement is shown. Inpractice, the CDMA transfer stations are placed so as to providecoverage whereby a plurality of subscribers 88, 90 are always withinrange of an A, B and C antenna. For example, CDMA transfer stations 76and 82 are antenna A type, while CDMA transfer station 80 is an antennaC type and CDMA transfer station 78 is an antenna B type. Thus,subscriber 88 receives signals from CDMA transfer stations 76, 78 and80, while subscriber 90 may receive signals from CDMA transfer station82, 78 and 80.

A time slot structure for use in the present invention is shown in FIG.7. Six time slots are used. Time slots 1 and 2 are used to receive,followed by time slot 3 wherein the subscriber station transmits,followed by time slot 4 also used for receiving. During time slot 5 and6 the CDMA receiver scans the transmission from other transfer stations.

Call Establishment

When a circuit is to be established or transferred, the base stationassigns a base station and transfer station frequency pair, a slot and aPN sequence. It then transmits to the transfer station all of theseassignments and identifies which subscriber is to use the circuit.During call setup, the transfer station passes on to the desiredsubscriber station, the slot and PN sequence assignments. For example,see FIG. 17 where the TDMA time slots 1 through 8 are associated withusers A through F, respectively. In a given time slot, e.g., time slot2, the message to user B contains synchronizing information 1701, commoncontrol data 1702 for system wide functions, private control data 1704and dedicated user traffic 1705 for user B. The dedicated user traffic1705 is used during call setup to transmit signaling information andinitialization data.

Forward Path

Signal compression and decompression, plus added bits for forward errorcorrection (FEC) is performed at the base station. In the forwarddirection, (to the subscriber station), the base station transmitscontinuously but the information in each slot is directed to aparticular subscriber station.

By way of example, the base station may transmit the information duringslot 1 on frequency fa. The transfer station receives the information bydemodulating the signal on frequency fa during slot 1, and regeneratingthe information only at the symbol or bit level. The transfer stationdoes not perform any decoding (i.e., error correction, compression ordecompression). The transfer station design is thus simplified byaccepting the already coded signal from the TDMA base station. Afterregeneration at the symbol level, the received TDMA signal is combinedwith the assigned PN sequence and retransmitted from the transferstation as a CDMA signal on frequency fp without any intentional delayto antenna A. The transfer station further stores the informationreceived from the base station in a memory buffer. At the end of theantenna A transmission, the information bits stored in memory buffer aremodulated onto a continuation of the PN signal and broadcast through anappropriate transmitter to antenna B. Thus, the identical informationsignal using the same PN sequence, but incremented a fixed number ofchips, is transmitted at antenna B. The relative position, or phase ofthe PN sequence relative to the transmitted information is different. Atthe conclusion of the first repeat, information in the time slot bufferis read out a third time to provide a third repetition of theinformation, modulated by a continuation of the PN sequence, with stilla different phase, through an appropriate transmitter to antenna C.

Subscriber Station Processing

The subscriber station, using the correct CDMA code, receives duringeach of the three slots containing information signal repetition, sothat it receives three identical repeats of the data packet from threeantennas located in different locations. The subscriber station thencompares the three receptions and selects the one with the best qualitywhich may be based on bit error rate, phase distortion, signal to noiseratio, etc. Thus, spatial transmit diversity is achieved. Only oneantenna is needed at the subscriber station. The subscriber stationdemodulates and decodes the signal, performs error correction,decompression, etc. A maximum likelihood combiner may be used to combinethe power from all three time slots. Ideally, the energy of receiveddata packets is combined in a maximal manner before making a harddecision.

During the third time slot T3, the subscriber station transmits back tothe transfer station using a similar PN sequence as it received. The PNsequence may be the one derived from reception (after regeneration) orit can be locally generated on the basis of the original code receivedduring call setup. Since the subscriber station does not transmit duringthe same time period as it receives, no diplexer or notch filter isneeded. A simple T/R (transmit/receive) switch is used to switch theantenna between transmit and receive. Only one receiver is necessary inthe subscriber station to achieve three branch diversity. The threechains needed by a Rake receiver, are not needed in the presentinvention.

Furthermore, the benefits of triple time and space redundancy, with somefrequency protection provided by the expanded spectrum, are not obtainedby adversely affecting capacity. The three branch diversity typicallyachieves a reduction for deep fades of at least 10 dB (a factor of 10x).While the three transmitted repetitions of the same information signalincreases the interference level by a factor of 3 (about 5 dB), becausethe fades are 10 dB less, the transmitter power levels can be reduced bya factor of 10 (10 dB). Thus the overall amount of interference isreduced by a factor of 10/3 or 5 dB. Because the transfer station tosubscriber link is operated in a self interference mode that means thatabout three times as many simultaneous subscriber circuits can be usedthan if diversity were not used.

Return Path

In the reverse direction (subscriber station to transfer station), threereceivers are connected respectively to the three antennas at thetransfer station to provide conventional three branch spatial diversity.The same analysis regarding interference and the number of circuitsavailable, applies to transmission in the reverse direction as well asin the forward direction, except that the information is transmittedonly once and is received simultaneously on the three base stationantennas.

In addition to increasing the number of subscribers per unit frequency,the present invention is cost effective. First the subscriber stationneeds only one receiver. Second, it does not need a diplexer. Third, thetransfer station does not need to decode or re-encode any signals. Thenumber of subscribers per transmitter is the same, however, sincespatial diversity is used in the reverse direction, the number ofsubscribers per receiver increased. Conversely, the noise of thesubscriber station can be allowed to be higher if the full use of theincrease in the number of subscribers is not fully utilized.

The signal received by the transfer station from the subscriber stationis retransmitted (again with symbol or bit level regeneration butwithout decoding), from the transfer station back to the base stationwithout intentional delay during the same slot. As long as the slot iswithin the same TDMA frame or at least with one frame's duration of theslot used from the base station to the transfer station, no additionaldelay is incurred by the use of the present system.

Transfer Station First Embodiment FIGS. 8, 9, 15

The CDMA transfer station has a TDMA input at antenna T. The output sideof the transfer station at antennas A, B and C, uses a CDMA structure toreach a large number of subscribers in relatively densely populatedareas. CDMA possesses several attributes that make it desirable for thisapplication. The wideband signal is inherently robust in a multipathenvironment and it has the ability to overcome interference, intentionaland otherwise. The possibility that selective fading will cause theentire spectrum to be suppressed decreases as the transmitted spectrumincreases. A higher chip rate, or increased TW product, reduces theamount of fade margin that is required to achieve a specified level ofperformance.

Spread spectrum signals have inherent multipath protection to protectagainst fading. However, statistical models generally do not take intoaccount the frequency of occurrence or the duration of the fades. Thespecific geometry at each location, and how the geometry is changingwith regard to the receiver, determines the actual fading patterns. Forsmall cells, with low antennas, the difference in path length for strongsignals is very likely to be small. The result is flat fading. That is,the spectrum across ten or fifteen megahertz will fade at the same time.Therefore, it is not possible to use the inherent multipath protectioncharacteristics of spread spectrum signals to protect against flatfading unless at least 25 or 30 MHz of spectrum is available. Inaddition, there is often no multipath of consequence that would haveenough delay to gain an advantage from an additional Rake receiver. Evenso the use of real or artificial multipaths, requires additionalreceiver/correlators in the CDMA user terminal. Therefore, to maintainreliable operation using CDMA only, at least 15 dB of margin is requiredto be added to the link power allocation, particularly to account forthe situation where a mobile user stops in one of the nulls or a fixeduser shifts location geometry slightly.

The present invention utilizes the other important characteristic ofspread spectrum systems, the ability to overcome interference, as thetechnique to combat the difficult multipath situations. The capacity ofa CDMA system is limited by the amount of interference that is receivedby the desired receiver. As long as the TW product is great enough tobring the desired signal up out of the interference it doesn't matterwhat the transmitted data rate actually is. Therefore, with the presentinvention the transmitted information rate is increased to allow thetransmitted signal to be repeated three times from three differentantennas, thus obtaining transmission triple diversity which allows thetransmitted power margin to be reduced by at least 10 dB for a highperformance link. Therefore, even though additional interference isintroduced into the links, the CDMA processing gain readily overcomesthe adverse impact. That is, the gain from the triple diversity farexceeds, in a high quality system, the loss due to added interference.

A block diagram of a transfer station in accordance with the firstembodiment of this invention is shown in FIG. 8 for the forward channel.The TDMA antenna T, 916, is coupled through a transfer receive switch918, to a TDMA receiver 800. The output of the TDMA receiver 800 iscoupled to a demultiplexer 802, the output of which is stored in timeslot buffers 806. A time multiplexer 808 accesses the contents of thetime slot buffers 806 and provides data packets output to plural CDMAencoders 810 intended for antenna A transmission. The output of timemultiplexer 808 also provides data packets output to plural CDMAencoders 812 intended for antenna C transmission. Similarly, the timemultiplexer 808 provides data packets output to plural CDMA encoders 814intended for antenna B transmission. Each of the plurality of CDMAencoders 810, 812 and 814 are provided to respective CDMA transmitters816, 824 and 826. Each of CDMA transmitters is coupled to a respectiveantenna 822, 824 and 826 to provide respective antenna A, antenna B andantenna C transmissions.

The coordination of the timing and control of the TDMA receiver 800, aswell as the time slot buffers 806, the time multiplexer 808 and each ofthe plurality of CDMA encoders, is controlled by a synchronization andcontrol apparatus 804. The synchronization and control apparatus 804also provides a location identification (ID) representing the particulartransfer station to the plurality of CDMA encoders 810, 812 and 814 forinclusion on the transmitted signals at antennas A, B and C.

The transfer station of FIG. 8 also includes a CDMA receiver and TDMAtransmitter 900, which is shown in further detail in the block diagramof FIG. 9. The TDMA transmitter is coupled to antenna 916 throughtransmit receive switch 918, while the CDMA receivers are coupledthrough respective diplexers to antenna A, antenna B and antenna C, asshown in further detail in FIG. 15.

FIG. 9 is a block diagram of a transfer station illustrating thestructure of handling signals in the reverse channel. Antennas A, B andC, respectively shown as 822, 824 and 826 are coupled to respective CDMAreceiver A, 902, CDMA receiver B, 904, and CDMA receiver C, 906. Theoutput of the respective CDMA receivers A, B and C is fed to maximumlikelihood combiner 908, the output of which is provided to memorybuffers and time slot multiplexer 910. The memory buffers in time slotmultiplexer 910 provide data packets to a TDMA transmitter 914 which iscoupled through transmit receive switch 918 to antenna 916. The TDMAreceiver and CDMA transmitter 828 corresponding to the block diagram ofFIG. 8 is coupled to the other terminal of transmit receive switch 918.

FIG. 15 illustrates the antenna configuration of a transfer stationpermitting antenna A, antenna B and antenna C to be shared between TDMAand CDMA transmit and receive signals. Modulator 1502 is coupled througha time multiplexer 1503 to diplexers 1510, 1514, and 1518, respectivelycoupled to antenna A, 1512, antenna B, 1516 and antenna C, 1520. Theother input of diplexers 1510, 1514 and 1518 is respectively coupled tothe output of demodulator 1504, 1506 and 1508.

In the operation of FIG. 8, a TDMA signal received on antenna 916 isdemultiplexed and placed in time slot buffers 806. A data packetintended for a given subscriber is selected by time multiplexer 808during time slot 1 to encode a CDMA signal by one of plural encoders 810for transmission on antenna A. The same data packet is again selected bytime multiplexer 808 to encode a CDMA signal by one of plural encoders812 during time slot 2 for transmission on antenna B. Finally, the samedata packet is subsequently selected by time multiplexer 808 to encode aCDMA signal by one of plural encoders 814 for transmission during timeslot 4 on antenna C.

In the reverse direction, and in reference to FIG. 9, the CDMAtransmission from the subscriber station during time slot 3 issubstantially simultaneously received on antennas 822, 824 and 826. Eachof the CDMA receivers 902, 904 and 906 receive the same data packet. Amaximum likelihood combiner 904 combines the power from all three timeslots before making a hard decision. Generally speaking, the signalwhich is strongest and error free will be selected. After selection, thedata packet is held in a memory buffer and time slot multiplexer 910waiting to be placed in its appropriate time slot for transmission byTDMA transmitter 914 on antenna 916.

Transfer Station Second Embodiment FIG. 12

A transfer station in accordance with the second embodiment of thepresent invention is shown in FIG. 12. In essence, this transfer stationis similar to the transfer station of FIGS. 8 and 9 except that only oneCDMA antenna, A, B or C, is provided. In particular, in FIG. 12 antenna1200 is coupled through a transmit receive switch 1202 to a TDMAreceiver 1204. The output of the TDMA receiver 1204 is demultiplexed in1206 and placed in time slot buffers 1208. A data packet placed in timeslot buffer 1208 is time multiplexed by multiplexer 1210 to one of aplurality of CDMA encoders 1212. The encoded CDMA signal is amplified inCDMA transmitter 1214, coupled through diplexer 1218 to antenna A, 1228.

Antenna A 1228 also operates to receive CDMA signals. Towards this end,a CDMA receiver 1226 is coupled to antenna A, 1228, through diplexer1218 to provide received data packets in combiner and time slot buffers1224. A time multiplexer 1222 takes the data packets in time slotbuffers 1224 and composes a time multiplex signal to TDMA transmitter1220 which is coupled through transmit receive switch 1202 to antenna1200. The operation of the transfer station is controlled by asynchronization and control apparatus 1216 which also includes uniquelocation identification (ID) for this particular transfer station, andcall setup control parameters.

In operation, the transfer station receives TDMA signals on antenna T,1200 which are demodulated in TDMA receiver 1204, and demultiplexed indemultiplexer 1206 for placement in time slot buffers 1208. The datapackets in time slot buffers 1208 are transmitted on antenna A duringtime slot 1. Towards this end, time multiplexer 1210, CDMA encoders 1212and the CDMA transmitter 1214 retrieve the respective data packets fromtime slot buffers 1208 and encode the appropriate data packet in a CDMAencoded signal on antenna A. On the return path, CDMA receiver 1226receives signals simultaneously on antennas A, B and C during all timeslots. The received data packets are demodulated by respective PN codes,and placed in time slot combiner buffers 1224, each time slot assignedto a different user. Thereafter, data packets are time multiplexed inmultiplexer 1222 for transmission by the TDMA transmitter 1220 throughthe transmit receive switch 1202 on antenna 1200.

The Transfer Station is the conversion point for mapping the TDM/TDMAsignal into a CDMA signal. The CDMA signal, when designed properly hassuperior performance against multipath interference. The input side ofthe transfer station is part of a structured distribution network. It isbasically a tandem relay point in the network, that is, the address tothe final CDMA user also includes the address of the intermediary point(the transfer station). Since, in the general case, the final CDMA usermay move and access the network through another transfer point it willbe necessary to provide the ability to enter the transfer stationaddress independent from the CDMA user's address. For fixed subscriberssuch as the TDMA subscriber station 40 in FIG. 2, this will not be anissue except for backup routing or for fade protection.

The preferred input network includes a number of base stations, transferstations and TDMA user stations as shown in FIG. 2. Any time slot on anyfrequency could be assigned to any TDMA user or transfer station. Toreduce the cost of the transfer station it is proposed that once a CDMAuser is connected through a specific transfer station any additionalCDMA users, assigned to that transfer station, also be assigned to atime slot on the same frequency as the first user. By properly managingthese assignments the number of TDMA radio elements can be reducedsignificantly. The base station 24 or the switching center and centralprocessor 22 will manage the radio resource and assign the frequencies,time slots and the PN codes, thus assuring efficient use of the spectrumand the radios. The frequency, time slot and PN code are all assignedduring the initial call setup process.

The local transmissions on the output side of the transfer station areCDMA, but each subscriber is assigned a specific time slot of a timedivision signal. Therefore, the individual information rate is increasedby the number of time slots. However, the total data rate for allsubscribers stays the same and the total transmitted power for allsignals remains the same, it is just redistributed. Since the individualtime slots are turned off unless there is activity the transmitted poweris reduced by approximately 3 dB for voice traffic. Because the sameinformation is transmitted three times the average transmitted power isincreased by 5 dB. Therefore, the total transmitted power from eachtransfer station is increased by 5 dB, transmitting three times, butalso reduced by 10 dB, diversity improvement, resulting in a 5 dBoverall reduction in average power. Overall, the interference introducedinto other cells is reduced by 5 dB.

The base station (24 in FIG. 2) or the switching center and centralprocessor (22 in FIG. 2) will also manage the handoff process. Therewill have to be at least four time slots to obtain diversity on the CDMAside and still have a time slot for the CDMA receiver to scan othertransfer stations. Four time slots only provide dual diversity. Withfive time slots it is possible to achieve the desired level of triplediversity. Of course, by adding additional receivers in the CDMA user'sterminal it will be possible to scan in parallel for better synchsignals. However, adding another receiver in all the CDMA user'sterminals would be an expensive solution. Therefore, with three timeslots there is only dual diversity and no handoff. With four time slotsthere is triple diversity for fixed CDMA subscribers and dual diversityfor mobile CDMA subscribers. With five time slots there is triplediversity for both fixed and mobile CDMA users. With six or more timeslots there is the opportunity to add flexibility to the channelstructure. FIG. 7 shows the CDMA user terminal slot structure for sixtime slots.

The triple antenna structure at the transfer station is used on thereturn link by simultaneously listening to a single burst from eachactive subscriber, in his assigned time slot, on all three antennas,thus also achieving triple space diversity. The overall timing structurefor the forward and reverse CDMA links, at the transfer station, areshown in FIG. 10A. For illustrative purposes six time slots have beenshown, but as described previously any number of time slots, three ormore, can be implemented, the upper reasonable bound being in theneighborhood of 32.

The order of transmission of the three active time slots can bedistributed over the total number of time slots, and even more thanthree time slots could be used. With triple diversity the powertransmitted from the CDMA user terminals can be reduced by at least 5dB, probably more, but 5 dB is in keeping to match the performance ofthe forward link. In any case, the transmitted power is controlled andkept at the minimum level to maintain a high quality link. It is alsopossible, at higher frequencies, to achieve some antenna independenceeven on a relatively small radio or area. Therefore, a similar approachof the transmission space and time diversity, that is used on theforward link, may also be applied to the reverse link. Dual diversityshould yield a significant improvement for most situations.

Each transfer station continuously transmits a spread spectrum channelfor synchronization and control purposes. The synchronization andcontrol channel identifies the particular transfer station and managesthe user terminals as long as they are assigned to the transfer station.A large portion of the time the synchronization and control channel doesnot carry any user traffic. The synchronization and control channel canbe a narrow band channel that can be easily acquired and tracked. Theinformation bearing portion of the control signal has a preassigned timeslot and includes system and signaling messages to all the usersassigned to the particular area covered by that transfer station. Theprocessing gain is sufficient to allow a transfer station to includeseveral time slotted CDMA signals to be transmitted in parallel, thusallowing the antenna array to be shared. Also, only one synchronizationand control channel is required for multiple slotted CDMA modules thatare integrated at a single location.

Subscriber Station FIG. 13

A block diagram of the subscriber station in accordance with the presentinvention is shown in FIG. 13. Antenna 1300 is coupled to CDMA receiver1304 through transmit receive switch 1302. The output of CDMA receiver1304 provides data packets to data buffers 1306, 1308 and 1310. Acombiner 1314 selects and combines the data held in buffers 1306, 1308and 1310 to provide an output to a digital to analog converter 1316,which also includes means for decompressing the compressed signal toprovide an audio output. An analog audio input is provided to analogdigital converter 1322, which also provides means for compressing theaudio signal. The output of the analog to digital converter 1322 is adigital form of audio samples assembled as data packets in memory buffer1320. A CDMA transmitter 1318 encodes the contents of memory buffer 1320and provides a CDMA encoded signal through transmit receive switch 1302to antenna 1300. The CDMA subscriber station is synchronized by asynchronization and timing controller 1312, which also measures signaldelay for location measurement, described below.

In the forward direction, CDMA receiver 1304 receives three identicaldata packets placing one of the data packets during time slot T1 inbuffer 1306, a second of the data packets during time slot T2 in memorybuffer 1308, and a third data packet received during time slot T4 inmemory buffer 1310. The combiner 1314 selects one or more of thecontents of the memory buffers to be combined or selected as the bestreceived data to be converted to an analog audio output of the output ofdigital to analog converter 1316. By using three time and spacediversity data packets, the present system is less susceptible to fadingand since the same receiver is used to demodulate all three samples, nocomplex signal strength balancing process is required.

In the reverse direction, the analog audio input to analog to digitalconverter 1322, which also includes a digital compression algorithm,provides a data packet to buffer 1320. During time slot T3 the CDMAtransmitter 1318 encodes the contents of buffer 1320 for transmission asa CDMA signal on antenna 1300.

The simplification of the CDMA user terminal is a major consideration inthe present system. The main simplification is the ability to time sharethe receiver, and particularly the correlator as it performs itsdifferent functions. The ability to transmit and receive at differenttimes also simplifies the implementation of the small portable userterminal. The single receiver sequentially receives the three spacediversity signals in the three different time slots and then moves todifferent codes to look for improved signals from other transferstations. The same receiver is also used for the purpose of acquisitionand tracking. Since the user terminal does not receive during the slotwhen it is transmitting there is no need for a diplexer and notchfilter. Only a simple on/off switch is used. Since only one PN code isneeded at a time, the PN code generation process is also greatlysimplified. The baseband processing can be accomplished on a relativelylow speed common processor.

In those time slots where the user terminal is not receiving ortransmitting the receiver is free to look for the synchronization andcontrol channels from other transfer stations. When the user terminalidentifies a synchronization and control channel that is better than theone he is assigned, the user terminal sends a message to the networkcontroller telling the controller that he has identified a potentialcandidate for handoff. The network controller uses this input, alongwith other information, to make the decision to handoff. The networkcontroller sends the handoff message to the affected entities. Theidentity of the codes that are to be searched by the user terminal areprovided by the network central controller through the transfer stationwhere they are placed on the control channel.

Time Slot Structure FIGS. 10 a, 10 b, 11 a, 11 b, 17

The time slot assignment for multiplexing 6 simultaneous calls is shownin FIG. 10A. Time slot assignments for transmission 1002 and forreception 1004 are illustrated. The entry in each box contains theactivity during the corresponding time slot. During time slot 1, antennaA transmits T1 to user 1, antenna B transmits T6 to user 6 and antenna Ctransmits T4 to user 4. At the same time, antennas A, B and C receive R5from user 5. During the next time slot 2, antenna A transmits T2 to user2, antenna B transmits T1 to user 1 and antenna C transmits T5 to user5. At the same time antennas A, B and C receive R6 from user 6.Continuing across the diagram in FIG. 10A, during time slot 3, antenna Atransmits T3 to user 3, antenna B transmits T2 to user 2 and antenna Ctransmits T6 to user 6. At the same time antennas A, B and C receive R1from user 1.

Note that during time slot 3, none of the antennas A, B or C istransmitting to user 1. Instead, user 1 is transmitting and the transferstation is receiving on all three antennas from user 1. However, duringtime slot 4, the third transmission to user 1 is transmitted. That is,during time slot 4, antenna A transmits T4 to user 4, antenna Btransmits T3 to user 3 and antenna C transmits T1 to user 1. Time slots5 and 6 are not directly used for data transfer to or from user 1. Thetime slot assignments shown in FIGS. 10A, 10B, 11A and 11B areconsistent with FIG. 7, wherein user 1 receives during time slots 1, 2and 4, and transmits during time slot 3. The pattern can be seen in FIG.10A slot assignments by looking for times when T1 is transmitted.Transmission of T1 appears in time slots 1, 2 and 4, on antennas A, Band C respectively. No transmission to T1 appears during T3, butreference to receiving time slots 1004 indicates that R1 is receivedfrom user 1 during time slot 3. Since in any given time slot, there arethree transmissions and one reception simultaneously, at least 4addressable CDMA PN spreading code sequences are required.

Thus, time division multiplexing is used in the sense that successivetime slots carry data directed to different users. Code divisionmultiplexing is used in the sense that during each time multiplexed timeslot, multiple PN code sequences permit simultaneous communication withmultiple users. The result is a time division multiplexed, code divisionmultiplexed signal.

The time slot assignment for multiplexing 12 simultaneous calls is shownin FIG. 10B. Time slot assignments for transmission 1006 and forreception 1008 are illustrated. During time slot 1, antenna A transmitsT1 to user 1 and T7 to user 7, antenna B transmits T6 to user 6, and T12to user 12, and antenna C transmits T4 to user 4 and T10 to user 10. Atthe same time, antennas A, B and C receive R5 from user 5, and R11 fromuser 11.

The time slot assignment for multiplexing 24 simultaneous calls is shownin FIGS. 11A and 11B. FIG. 11A shows the transmission from the transferstation (forward direction), while FIG. 11B shows the transmission tothe transfer station (reverse direction). Time slot assignments fortransmission 1102, 1104, 1106 and for reception 1108 are illustrated. Byway of example, during time slot 5, antenna A transmits T5, T11, T17 andT23 (i.e., T5 to user 5, T11 to user 11, etc.) Antenna B transmits T4,T10, T16 and T22. Antenna C transmits T2, T8, T14 and T20. At the sametime, (during time slot 5), antennas A, B and C receive R3, R9, R15 andR21 (i.e., R3 from user 3, R9 from user 9, R15 from user 15 and R21 fromuser 21).

For FIG. 10A, one CDMA encoder per antenna is required to handle 6simultaneous calls. In FIG. 10B, two CDMA encoders per antenna arerequired to handle 12 simultaneous calls. Similarly, in FIG. 11A, fourCDMA encoders per antenna are required. Thus, for example, if 180 PNcode sequences are available, then 180/6 or 30 CDMA encoders per antennaare required to handle 180 simultaneous calls. If, for these largernumber of required accesses, the number of time slots is increased, thenumber of encoders will decrease proportionally.

Alternate System Configurations FIGS. 14, 16

A further enhancement extends the distance between the transfer stationdiversity antennas by using broadband cables that are a thousand feet ormore. The transfer station sends the final radio frequency spreadspectrum signal down the cable to the antenna. The antenna at the end ofthe cable contains a radio frequency amplifier. An implementationdistributing signals by cable has the same improvement against blockageas described for the multiple transfer station transmission diversityapproach.

However, instead of using a separate cable for each antenna, a preferredembodiment shares a single cable and uses frequency multiplexing toassign a different cable carrier frequency to each antenna. Thus, thedesired signal is only transmitted from the antenna nearest to the userwhich reduces the interference. As a further enhancement, a cabledistribution system integrates different elements into a local personalcommunications system network. The basic building block is the six timeslotted CDMA module that serially drives three antennas to obtain tripletransmission space and time diversity. For the sake of simplicity, thedesign of the transfer station handling the incoming TDMA signal alsohas a basic six time slot structure. The six time slot modularity canreadily be deployed to accommodate multiples of 12, 18, 24, and 30 or32. FIG. 14 shows the implementation for several different combinations.The preferred embodiment utilizes a wireless input, such as W or WE, asthe input to the transfer station, however, a cable distribution systemworks equally well with hard wired signals as the input.

In a cable based personal communication system, the transfer stationsare moved back to the central controller, which reduces the cost of thetransfer station since it does not have to be ruggedized or remotelypowered. It also reduces the number of spares required and the cost tomaintain the units since they are all in one place and easy access. Thetransfer stations can also be dynamically reassigned as the traffic loadchanges during the day or week, thus significantly reducing the totalnumber of required transfer stations. The bandwidth of the distributionnetwork increases, but developments in cable and fiber opticdistribution system have increasing bandwidth at falling cost toaccommodate the increase in bandwidth at reasonable cost. The advantageof having several interconnection options to select means that thechoice of interconnection becomes an economic choice determined by thecost factors associated with each installation. Each network is expectedto include many or all of the interconnection options.

The system arrangement in which the transfer stations are moved back tothe same location as the central controller, is depicted in the lowerportion of FIG. 14. A general two-way cable or fiber optic widebanddistribution system 1402 is used to link the centrally located transferstations to the remotely located antennas. Considerable flexibility inconfiguring the wideband spectrum into signal formats is available forlinking the centrally located transfer stations to each transfer stationantenna. However, for simplicity it is preferable to retain the TDMAprotocol with its time slotted CDMA triple space/time diversity airinterface protocol, and frequency translate signal as a common airinterface to each antenna.

Each antenna is assigned a separate center frequency on the widebanddistribution cable 1402. Due to the TDMA and CDMA sharing ability, manyusers can be served on the same antenna using the same cable frequency.The transfer station antenna at location N, includes a transceiver whichis tuned to the assigned cable frequency. The central controllertransmits and receives data packets in the final TDMA/CDMA waveformrepresenting telephone traffic on each assigned frequency of thewideband distribution cable 1402. Thus, as shown in FIG. 16, each remotelocation includes a remote transceiver (transmitter, receiver, localoscillator, diplexer and antenna) at site 1602. The remotely locatedunit is a relatively simple receiver, frequency translator and low powertransmitter, for both the forward and reverse directions. A low powertransmitter amplifier is suitable because the cells are small and triplediversity (three antennas and three time slots) is being used to linkthe subscriber station to the system. The transmit side of the centralcontroller provides individual information flows along with theassociated signaling and control information at interface A′ in FIG. 14,which is presented in assignable time slots in the form of packets.

The signaling information includes the called parties identificationnumber(s), code, service profile and authentication code, etc. Thecontrol information includes routing information (i.e. which basestation, transfer station, antenna designation), power levels, trafficon or off, handoff messages, etc. A large amount of this information istransmitted before the user information (telephone voice traffic) startspassing over the circuit, however, a significant amount of informationis also passed during the time when telephone voice traffic is actuallyon the circuit. A separate control channel is required even after theconnection to the user has been completed. The base station functiontranslates this information into the protocol that is required tointerface to the TDMA air interface and provides a TDMA radio spectrumat interface W. The transfer station converts the TDMA protocol to atime slotted CDMA triple space/time diversity air interface protocol andtransmits this signal first on antenna A, then on antenna B and finallyon antenna C (FIG. 14).

The centrally located combined base station and transfer station (B-T)module 1404 combines the base station and transfer station function andconverts the signal appearing on A′ to the time slotted CDMA triplediversity air interface. A B-T combined module may be achieved by directcombination of separate equipment, or the modules developed for thecombined base station and transfer station use can be integrated. TheCDMA signal branches at the output of the transfer station or at theoutput of the B-T module as shown in FIGS. 15 and 16. In the case of theof the transfer stations which are connected to respective antennas bythree different cables, the output is just switched at the appropriatetime. When one cable is used to reach all the antennas the output of thetransfer station is frequency hopped at the appropriate time by changingthe synthesizer frequency to the assigned frequency of the antenna. TheB-T module is similarly frequency agile.

It is important to note the user information is replicated in each ofthe three time slots, but the PN code continues to run and is differentduring each time slot. Therefore, the repetition is not the same as inthe case of imitation multipath or emulated multipaths. The PN generatorjust keeps on running without storing or resetting the sequence. Runningthe PN code continuously is simpler to implement as compared to startinga PN sequence anew.

In the foregoing discussion, it is assumed the time slots follow oneright after the other; this is not necessary, however, as long as thereceiver has a priori knowledge of the hopping sequence. In thepreferred embodiment, the B-T transmits on two contiguous time slots andthen listens to the response signal from the user terminal. During theuser transmission time slot the user terminal tells the B-T module tonot send the third diversity time slot if the first two time slots havegiven adequate performance and location measurement is not needed. Theuse of only dual diversity reduces the interference to the other users,and frees up the user receiver to perform other functions.

An alternate approach is to utilize a ⅓ forward error correcting codethat is spread over all three time slots. The use of such codingprovides improved performance if the error statistics during each of thetime slots are nearly the same. If one time slot becomes significantlyworse, and it can be identified as being bad, it may be better to ignorethe bad time slot and request an antenna handoff to replace that timeslot if the poor performance continues. Since it is expected that thereal diversity channel statistics will result in unequal time slotstatistics, the preferred alternative is to not use a forward errorcorrecting code over the three time slots. Even though error detectingand correcting codes are only included within each time slot, forwarderror correcting codes may be used over multiple time slots.

Each antenna, assuming there is data to transmit, transmits during eachof the time slots. Since the data is transmitted three times there willbe three CDMA signals transmitted in each time slot for each moduleassigned to that antenna. If there are 4 modules assigned to theantenna, 4 modules supports 24 users at any one time, there would be 12CDMA signals emanating from the antenna in each time slot, (see FIGS.11A, 11B). If the duty factor is approximately 50% then only six CDMAsignals will actually be transmitting and if 20 to 25% of the time thethird time slot is not required only 4 to 5 CDMA signals would betransmitted at a time. The same antennas are used for the receive side,or reverse link, (user to transfer station).

As stated previously the user CDMA terminal transmits only during onetime slot and the transfer station simultaneously receives thattransmission on the same three antennas resulting in receiver triplespace diversity. The three receive signals come into the transferstation, or B-T module, either on separate wires or at differentfrequencies, as shown in FIGS. 15 and 16, and are processed separately.These processed signals are summed together using maximum likelihoodcombiners. The S/I from each antenna path is measured and kept in memoryover an interval of at least ten time slots. The record of signalstatistics is used by the maximum likelihood combining process. Storedsignal statistics are also useful in the decision process for executinghandoff to other antennas.

The handoff process for the B-T cable network is based on the signalreceived from each of the antennas. The central processor receivesinformation on the quality of the links in both directions. On theforward link it receives information from the user CDMA receiveroperating on that link during an assigned time slot which is identifiedwith a particular antenna. On the reverse link it receives informationon the separate paths through different antennas. The information on thequality of paths through a particular antenna can be evaluated andcompared to other current paths through different antennas and withother new paths that the user terminal is continuously searching. When acurrent path in a particular time slot continues to deteriorate and abetter path is available the central controller assigns a new path(antenna) to the user terminal and notifies the user terminal it hasdone so.

The handoff process for the transfer station is similar except thehandoff is generally between transfer stations rather than antennas.When handed off from transfer station to transfer station all threeantennas associated with a particular transfer station are handed offwith the transfer station. A few transfer stations may be implementedwith widely separated antennas. In the case where there are transferstations with widely separated antennas the handoff process describedfor B-T module could also be used.

Operational Description: A new subscriber turns on his CDMA userterminal and scans the synchronization codes until he acquires asynchronization code. The CDMA user terminal then initiates aregistration message. The transfer station receives this message andpasses it to the central controller who acknowledges it with anacknowledgment message back to the user terminal. The central controllergoes to the home register of the new terminal and obtains the userprofile and places it in the file for active users. The new user is nowregistered and all calls will be forwarded to this new region ofservice.

There are 28 different synchronization codes and one synchronizationcode is assigned to each area. The 28 areas make up a region and thecodes are repeated in the next region. The transfer stations within anarea are given different shifts or starting points for their particularcode. Therefore, each transfer station, or widely separated antenna, hasan identifiable code. The central controller knows which antenna, ortransfer station, that the new user registered through so the controllerwill route all information to the new user through that node. Thecentral controller will also give the new user a set of codes, ordifferent starting points on his current code, to search for the purposeof identifying diversity paths or handoff candidates. The new usercontinues to monitor the synchronization and control channel during halfhis time slots. The other half of his time slots he scans for bettersynchronization channels.

The user is paged on the control channel and given a CDMA and time slotassignment which he sets up so he will be ready for the beginning of thecall. When the user requests service he is also given a CDMA code andtime slot assignment for the duration of the call. The user terminalremains in this state until the end of the call, unless the signal inone or all the diversity paths becomes weak. Since the user receiver iscontinuously evaluating the incoming signals and scanning for better newpaths, it will know if a path is going bad and will notify the centralcontroller of this condition along with a list of better candidates. Thecentral controller will order a handoff and the user terminal will go tothe new CDMA code and time slot. None of this activity is detectable bythe end user.

At the beginning of each time slot is a short unmodulated section,without user information, used for resynchronization and rangeadjustment, followed by a short control message section. These shortbursts are sent whether there is user information to be sent or not. Ifno user information is to be sent the control message confirms this andthe transmitter power is reduced by ten db. for the user informationportion of the time slot. It should be noted four time slots areavailable on the forward channel for passing user information dependingon what agreements have been established between the user and thecentral controller. These slots as described above can be turned off sothat other users have access to additional capacity. The multiple timeslots can be used for diversity improvement or sending increased datarates, multiple data channels or a graphics channel along with a voicechannel. The possibility of extending several parties on a conferencecall is also possible.

Location Processing FIGS. 20, 21, 22, 23

FIG. 20 shows the radio links of FIG. 1 or FIG. 4, where the car and itsantenna are represented by user antenna U. The radio links are timeslotted as shown in FIG. 10A. The radio link AU is time slotted and ispresent during time slot 1. Radio link BU is also time slotted and ispresent during time slot 2. Radio link CU is also time slotted and ispresent during time slot 4. Radio link AU establishes the absolute rangefrom U to antenna A. The range to antenna A forms a reference to measurethe difference in path lengths between radio links AU and BU. Similarly,the path length of radio link AU is also used as a reference to measurethe difference in path lengths between radio links AU and CU.

Since the time occurrence of the all ones vector (for synchronization)is the same at all three antennas, the ranges to all three antennas maybe derived from the difference in respective arrival times of the allones vector within each time slot. The location center, having thephysical geographic coordinates of all three antennas, calculates thelocation of the user's antenna U.

The geometry of location determination is shown in FIGS. 20, 21, 22 and23. The first range measurement AU establishes the user as someplace oncircle A in FIG. 21. The second range determination establishes the useras also being someplace on circle B. The only locations this can be trueis where the circles intersect each other at points X and Z. Therefore,his location has been narrowed down to two possible points. The thirdrange determination establishes the user someplace on circle C. Sincethe user is also on circle C, he must be at point Z. Obtainingadditional ranges to other antennas confirms the first set ofmeasurements and in many cases improves on the accuracy. If the terrainhas significant variations in height the constant range circles becomeconstant range spheres and the extra measurements remove any ambiguitythat could be caused by adding the third dimension. The positionlocation processing center converts these coordinates into user friendlyinstructions. Range measurements by the CDMA system are obtained asfollows:

1. The pseudo noise code as it is stretched out between A and U to actas a yardstick. The time required to propagate between A and U allowsmany chips, the propagation time in microseconds times the chip rate inmegachips, to represent the length of the link or be “stored” in thelink during signal propagation. See FIG. 20.

2. There are two ways to increase the number of chips stored in thepropagation path. One is to increase the path length and the other is tospeed up the chip clock rate. Increasing the chip clock rate isanalogous to marking a ruler in a smaller scale. Therefore, increasingthe chip clock rate stores more chips in the path delay and makes itpossible to make more accurate measurements.

3 The path length from antenna A to user terminal U and back to antennaA, can be measured by transmitting from A, then retransmitting the samePN code, with the arriving phase, from user terminal U, and comparingthe repeated signal as it is received back at antenna A to the signalthat was previously transmitted from antenna A. By delaying the originalsignal until it matches, chip by chip, the received signal, at A, andcounting the number of chips that are slipped, the total delay isproportional to twice the distance between antenna A and antenna U.

4. The accuracy of the distance measurement is approximately ¼ of thenumber of feet represented by one chip. The ¼ chip is an implementationconstraint determined by how precisely the correlation peak is detectedand tracked. It is possible to reduce this error by autocorrelationtechniques, but ¼ chip is a realistic resolution.

5. To determine the path length between antenna A and user terminal U,described in paragraph 3 above, FIG. 22 shows the signals 2202transmitted and signals 2204 received at antenna A. At a chip clock rateof 10 megachips per second, there are approximately 100 feet representedby each chip. The delay of 51 chips between transmitted 2202 andreceived 2204 signals represents the time required for a radio wave totraverse a round trip between the subscriber station and the transferstation. One half of the round trip delay, or 25.5 chips represents thedistance to the antenna. Thus, the distance from antenna A to userterminal antenna U for the example in FIG. 22 is (51.times.100)/2=2550feet. The distance measurement accuracy is plus or minus 25 feet (100feet/4).

6. Thus, the distance AU is measured quite precisely. As describedpreviously the receiver uses a single receiver for all time slots. Whilethe subscriber receiver is listening to time slot one it is working inconjunction with the base station, to repeat the received waveform, samephase with no delay through the user terminal. The base stationreceiver, as described above, compares the received phase with thetransmitted phase to determine absolute range. The base station thentransmits the range value, thus measured, to the user terminal where itis stored for future retrieval and use. As noted above it is thewaveform phase that is important, if the starting point, the all onesvector, is maintained through the user terminal, a new similar PN codemay be substituted on the reverse link. A similar code could includethat same code shifted by a defined offset.

7. The same forward and return measurement process described above,could be used to obtain the other two ranges (to antennas B and C) withthe results also stored in memory at the user station. However, directrange measurement to all three antennas is not necessary. See FIG. 23.The same receiver retrieves information over all three paths. In sodoing, the receiver adjusts for the difference in path length at thebeginning of each time slot. Once the adjustment is accomplished, on thefirst time the receiver uses this antenna as an information channel, thecode is stored and retained in memory until the radio returns to thistime slot whereupon, it is taken from the memory and used as thestarting point for the tracking loops. Therefore, the receiver isessentially maintaining three separate sets of receiver parameters,emulating three different receivers, one set of parameters for time slot1, a different set for time slot 2 and still a different set for timeslot 3. The distances to antenna B and antenna C can be determined byadding or subtracting the offset, measured in chips, from the absoluterange value measured on link AU. Actually the offset is determinedbefore the time slot is used for the first time as an informationchannel, this determination is made in the process of looking for newpaths for handoff. The delay and measure of signal quality is determinedand maintained in the potential handoff targets file. These delay offsetmeasurements are also used as additional range measurements in theposition location process.

In particular, continuing the above example, the signal 2302 transmittedat antenna A represents a range of 25.5 chips from antenna A to userterminal antenna U. Signal 2304 received at antenna U from antenna A isused as a reference to measure the relative time of arrival of signalsfrom antennas B and C, adjusted for the different time slots in whichthese signals are placed.

Since timing for time slots 1, 2 and 3 is sequential, the real time chippatterns for slots 2 and 3 do not overlap. However, after adjustment fortime slot delays, the timing relationship is as shown in FIG. 23. Thusadjusted for the time slot difference, signal 2306 received from antennaB at user terminal antenna U, is received in advance (i.e., offsetrelative to the signal from antenna A) by 8 chips. Similarly, signal2308 received from antenna C at user terminal U, is also received inadvance (i.e., offset relative to the signal from antenna C), but by 6chips. Received signals may be either delayed or advanced (i.e., have apositive or negative delay) relative to the reference signal 2304.Receipt in advance indicates that the antenna (B or C) is closer thanantenna A. Conversely, a delayed receipt indicates that the antenna (Bor C) is further away than antenna A.

In FIG. 23, the range from antenna B to antenna U is 25.5-8=17.5 chips.In feet, 17.5 chips is 17.5.times.100=1750 feet, the length of path BU.The range from antenna C to antenna U is 25.5−6=19.5 chips. In feet,19.5 chips is 19.5.times.100=1950=path length CU. The user terminal maybe located at Z, the intersection of circle A at 2250 feet from antennaA, circle B at 1750 feet from antenna B and circle C at 1950 feet fromantenna C.

In the alternative, location measurement may be accomplished bycomputing the intersection of two hyperbolas. The first hyperbola is thelocus of all points having a fixed difference in distance from two foci,which is proportional to the difference in delay between antenna A andantenna B. The second hyperbola is the locus of all points having afixed difference in distance from two foci, which is proportional to thedifference in delay between antenna B and antenna C, (or antenna A andantenna C). Antennas A and B are the foci of the first hyperbola, whileantennas B and C are the foci of the second hyperbola. In such manner,subscriber location may be computed without requiring a two way exchangebetween the user terminal and the transfer station to establish a firstrange measurement.

Location Services FIGS. 18, 19

Since the subscriber station receiver is receiving information overthree different paths that emanate from known locations, positionlocation information is derived by measuring the time of arrival ofmessages relative to a fixed time reference. The measurement accuracydepends on the chip rate, but at a chip rate of 10 megachips per secondit is quite accurate. There are several ways location measurement anddisplay can be accomplished, depending on how much processing isavailable in the user terminal. The choice also depends on who willactually use the information. It could be fairly passive, using only therelative chip offset information and obtaining a reference from thecurrent cell. The user could locally derive and display his location,similar to using a GPS satellite. A GPS receiver displays longitude andlatitude reading. Location information may also be sent back to aprocessing center that provides a service to the user. The processingcenter converts the longitude and latitude coordinates into a locationhaving geographic meaning, such as, a block number on a specific street.

Local geographic position measurement is particularly attractive topeople concerned about security and health problems. The manager of theservice center could either notify the police, family designate or theservice center could include, as part of a special service rate, thestaff to check on irregular circumstances. Of course, the service centercan also, for a nominal fee, tell an individual his street location andgive instructions on how to get to a desired destination address. Theseservices can be provided to users who are pedestrians or moving along invehicles. The destination instructions can be in the form of a set ofone time detailed directions, or specific and continuous intersectionprompting as the user travels the suggested route. The prompting couldtake the form of a voice command, or text display, telling the user toturn right at the next intersection. A delivery truck, cab, ambulance orfire truck could have a special screen that showed a local map withinstructions written on it. The instructions can also be modified as thetraffic congestion changes. The benefits of the present system are asignificant increase in public safety, convenience and productivity.

In the system configurations described previously, the separationbetween antennas is made sufficient to yield an accurate positionlocation capability. By positioning the antennas to obtain independentpaths sufficient to avoid flat fading due to interfering obstacles, thenthe separation is also sufficient to reduce the triangulation error to avery small number. The incremental cost of including optimization for alocation capability is nominal.

Position location processing is accomplished by a third party providerwhich owns and manages the position location center. Location servicecan be accomplished in several ways. The preferred approach is to makethe user terminal the repository for all location information bybuilding and maintaining a location file. The position location centerqueries the user terminal over the normal public switched telephonenetwork (preferably packet) when it needed information. Preferably, aprovision for encryption during transmission and an access code forprivacy is used. The user terminal could also send location informationto the location center, also over the public switched telephone network,responsive to user activation. For instance, when the user pushed analarm button, the radio sends the alarm message, along with the locationinformation, to the location center. The location center would respondaccording to prearranged directions and the level of subscribed service.Since the user terminal radio develops the code offset informationinternally, the only additional information the cellular system needs toprovide to the user terminal is the distance, one way or round trip,from the user to one of the base station/antennas. The distanceinformation, which would be provided as a service feature to the user,must identify the base station/antenna. All the measurements must beperformed within a time window of 100 milliseconds or the error as aresult of vehicle movement between measurements could become excessive.For stopped vehicles or pedestrians the time window to perform locationmeasurements could be much longer since there is little or no movementbetween measurements. Therefore, the distance measurement sent by thesystem to the user terminal includes the distance in feet, the time inmilliseconds and the identity of the measuring entity. Upon receipt ofthe distance message the user terminal stores the message and makes codeoffset measurements to several different antennas, and, if signal levelsare adequate, stores the composite information in the location file. Thelocation file is retained until a new distance message is received bythe user terminal radio, whereupon the user terminal radio again makesthe code offset measurements and updates the location file.

When the location center queries the user terminal radio as to itslocation, the radio sends the contents of the location file. Thelocation center processes this data into very accurate map data,position on a particular street (can be displayed on a typical streetmap). The system measures distance to the subscriber normally once everyminute when the subscriber is in the active receive mode, receiver on,waiting to be paged. The period between measurements is variable and canbe adjusted according to the needs of the user. The system sends thisnew distance to the subscriber station which places it in the file andenters new code offset measurements with it. If the subscriber isengaged in a conversation, the user terminal is transmitting, the basestation makes a measurement every ten seconds and if the distancechanges more than one hundred feet the system sends a message to thesubscriber station. Whenever the user terminal receives a distancemeasurement it adds the local code offset measurements and updates thefile.

It can be seen the user terminal's location file is updated at leastevery minute and more often if warranted. Therefore, the system can knowthe location of any active user within a distance of approximately 100feet. Better accuracy and more frequent updating is certainly possible,but due to the loading on the data links the number of subscribersreceiving higher performance should be the exception rather than therule. Whenever the user presses the alarm button on his portableterminal, the terminal transmits the contents of the location file threetimes which is long enough for the system to read a new distance andsend a message to the user terminal. The user terminal makes severaloffset measurements and sends the new location file three times. Thealarm message is repeated every thirty seconds until the battery goesdead. The user terminal radio can have a module added (with its ownbattery) that emits an audible tone whenever the radio alarm message istransmitted.

The system generates raw location information at the user terminal thatneeds to be converted into human readable map data. In general, thebasic longitude, latitude, or angle and distance readings are fine.However, there is a need for a third party to translate this data into aformat that is quickly usable by the mass public, as a service business.Since the user terminal has the basic location information, it can beprovided to any authorized entity that requests it from the userterminal. The location processing center periodically queries thesubscribed user terminals and maintains a file on their currentlocation. One potential service for subscribers with health problems isa monitoring system during exercise. If the subscriber stops in anunusual location for an excessive length of time and does not press thealarm button, the location center operator could request life signs orsend a medical technician to the paused subscriber. If there is anemergency, the location center operator knows the subscriber location inorder to send help. On the other hand, when the alarm button is pressed,the alarm message is addressed to the location center where they areequipped to handle such emergencies. The capability to track userterminals and provide help as the result of some action is useful formany applications. Tracking stolen cars, identifying congestion, keepingambulances from getting lost and reporting vandalism are but a fewexamples of the application of the present invention.

The system does, particularly in its distributed configuration asdescribed previously, require a consistent zero time reference acrossthe different base station antennas. Having a zero time referenceavailable significantly reduces the time to resynchronize as the signalhops from antenna to antenna and also aids in the search and handoffprocess. The location application capability described above allows thesystem to periodically perform a self calibration by placing several ofthe user terminals, as described above, at fixed locations anddetermining the proper zero time setting for these locations. By keepingthe correct answer in the central processor, as the system scans thesecheck points, it will get an error indication if the system is out ofcalibration. The same check points are used to show the effective delay,during the process wherein a variable delay is introduced byincrementing or decrementing the system delay in one or more of thesignal paths in the recalibration or adjustment process.

The calibration process could be easily automated. Automation could beimplemented in two ways. The first approach is to scan the check pointsevery minute and determine any error that has developed. If this errorreaches a significant level the communication system contacts thelocation center and provides the center with the corrections that needto be factored into the position location calculations. The latterapproach requires close coordination between the communication systemand the position location center. A more autonomous approach would bedesirable. The communication system itself could maintain the proper“zero” state by scanning the check points, as described above, and byhaving the ability to insert or remove delay 1806 in the path to theantenna.

FIG. 18 illustrates a system with self-calibration. Once every minutethe system queries each check point 1802. This results in a distancemeasure being sent to the check point 1802 where the check pointreceiver adds the code offset measurements and sends the contents of thelocation file to the processor 1804 where the received file is comparedwith a file that contains the correct measurements. If the differenceexceeds the threshold, the processor 1804 calculates the changes indelay that are required to bring the measurements within tolerance andpasses the correction to the controller. The controller maintains a filethat includes the variable delay 1806 to be inserted for each antenna.The controller changes the delay entry in the file and a new measurementis taken to validate the calibration. Changes that require significantchanges in delay are unlikely, but if this should happen, the controllerwould not initiate any measurements that include the leg that is underrecalibration. Thus, the position location capability also provides aservice for the communication system. Self calibration results in asignificant reduction in installation cost and allows the use of moreeconomical system components.

Location related communications between the antenna devices and thesubscriber terminal can be broken into several different links. Thefunctions that are performed by these different links are: 1, distancemeasurement (requires a two way link, but no traffic); 2, sendingmeasurement information to subscriber terminal (one way data link,except for possible retransmission requests); 3, measuring code offset(only requires user terminal to listen, no data is transferred); 4,Transmit location file to location center or communication processor1804 (data links can be either one way or two way). Distance measurementcan only be performed by the system and since it requires a two way linkit can be done while a normal conversation channel has been establishedor if the terminal is in the listening mode the system has to establisha short round trip connection.

The two way link is required because the base station measures the codephase difference between the signal it sends to, and the signal itreceives from, the user terminal. In FIG. 18 the foregoing function isaccomplished in processor 1804. In this sense, the system operates likea radar with a pulse the width of a PN chip. The one way data linkmessage transporting the distance message to the user terminal is asingle message that typically will include an error correcting code, andmay also require an acknowledgment message to be sent back from the userterminal to the base station. The acknowledgment message could be sentindependently or appended as part of the distance measurement function.

The code offset information is also placed in a file that is accessiblefrom outside the system. As described previously the user terminal timeshares one receiver on the three independent paths that emanate atdifferent times from the three different antennas. Therefore, thereceiver tracks three independent paths one after the other. The PN codeon each path is the same, and as described above the code has the samestarting time at each antenna, but because of the difference in distanceto the three different antennas, from the user terminal, the codesarriving at the user terminal are of different code phases. However,since the system cycles very rapidly from antenna to antenna, thereceiver cycles between signals received from each of the antennas.Therefore, the receiver maintains three separate starting states andtracking loops for the different time slots. At the end of each timeslot, the exact time is known in advance, the previous state is storedin the computer and restored at the beginning of the next time slotassigned to the same antenna. Thus, the processor is emulating threedifferent receivers. The receiver quickly adjusts for any slight driftthat occurred while the receiver was locked to the other antennas. Notethat the receiver has a specific starting state. Thus, the PN sequencehas been shifted to compensate for the difference in range on the pathbetween the user terminal and the first antenna and the path between theuser terminal and the second antenna. The difference is the code offset,because the code offset measures the difference in range. Thus, thedistance to the second antenna is known without having to do a closedloop (two way) measurement. The same process is followed for the thirdantenna.

Additional entries, greater than three, in the location file areavailable using the normal search mode that the user terminal radio usesto identify potential candidates for handoff. The user terminal radiosearches the pilot codes emanating from nearby antennas to determine ifany of these antennas have better signals than one of the three that arecurrently being used. If so, the user terminal notifies the system thata good candidate is available. The process of searching starts at thestate of the PN signal coming in from time slot number one and ifnothing is found at that state the radio adds a chip to the path lengthand integrates again. The radio keeps adding chips until it finds asignal or exceeds a range threshold. If it exceeds the range thresholdit resets the PN generator to a new pilot code and starts at the 0offset distance again. Therefore, when the radio finds a new pilotsignal it knows how many chips it added before it was successful. Theadded number of chips is also the code offset. The code offset valuealong with the identity of the code, which uniquely names the antenna,and the time stamp are entered into the location file. The radio placesthese entries in the location file even if they are not better than thecurrent signals. As the radio scans and finds new antennas it places thefour best results in the location file. As it continues to scan, olderentries are replaced with newer better entries.

Now that the necessary information is available in the user terminallocation file, it may be made available to any authorized requester.Location services may be provided by the communications operator or by acompetitive independent service provider. In addition, there will alsobe large private location centers operated by owners of large fleets.The location center 1902 receives the location files over the publicswitched network, see FIG. 19. The network can be a circuit switchednetwork or a packet switched network. A packet switched network isadequate and economical for this type of application.

1. A method for use in a subscriber unit, the method comprising:receiving a first signal from a first base station; receiving a secondsignal from a second base station; measuring a difference between afirst time of arrival of the first signal and a second time of arrivalof the second signal; and transmitting a third signal carryinginformation indicative of the difference between the first and secondtimes of arrival to a base station associated with one of the first andsecond base stations, wherein the third signal is spread coded.
 2. Themethod of claim 1, wherein the first signal is encoded with a firstpseudorandom code and the second signal is encoded with a secondpseudorandom code.
 3. The method of claim 2, wherein a portion of thefirst pseudorandom code carried by the first signal arrives at thesubscriber a number of chips before a portion of the second pseudorandomcode carried by the second signal, the number of chips corresponding tothe difference between the first and second times of arrival.
 4. Themethod of claim 1, wherein the first and second signals carry data. 5.The method of claim 1, further comprising displaying a location of thesubscriber unit, wherein the location of the subscriber unit is based onthe difference between the first and second times of arrival.
 6. Themethod of claim 1, further comprising generating an audible signal,wherein the audible signal indicates a location of the subscriber unitthat is based on the difference between the first and second times ofarrival.
 7. The method of claim 1, further comprising displayinginstructions for reaching a destination address.
 8. The method of claim7, further comprising modifying the instructions in response to changesin traffic congestion.
 9. The method of claim 1, further comprisingdisplaying a destination address.
 10. A code division multiple access(CDMA) subscriber unit comprising: an antenna configured to receive afirst signal from a first base station and a second signal from a secondbase station; and a circuit operatively coupled to the receiverconfigured to measure a difference between a first time of arrival ofthe first signal and a second time of arrival of the second signal;wherein the circuit is further configured to transmit a spread codedthird signal carrying information indicative of the difference betweenthe first and second times of arrival to a base station associated withone of the first and second base stations using the antenna.
 11. TheCDMA subscriber unit of claim 10, wherein the first signal is encodedwith a first pseudorandom code and the second signal is encoded with asecond pseudorandom code.
 12. The CDMA subscriber of claim 11, wherein aportion of the first pseudorandom code carried by the first signalarriving at the subscriber a number of chips before a portion of thesecond pseudorandom code carried by the second signal, the number ofchips corresponding to the difference between the first and second timesof arrival.
 13. The CDMA subscriber unit of claim 10, wherein the firstand second signals carry data.
 14. The CDMA subscriber unit of claim 10further comprising a display configured to display a location of thesubscriber unit, wherein the location is based on the difference betweenthe first and second times of arrival.
 15. The CDMA subscriber unit ofclaim 10 further comprising a speaker configured to generate an audiblesignal, wherein the audible signal indicates a location of thesubscriber unit that is based on the difference between the first andsecond times of arrival.
 16. The CDMA subscriber unit of claim 10further comprising a display configured to display instructions forreaching a destination address.
 17. The CDMA subscriber unit of claim16, wherein the display is configured to modify the instructions inresponse to changes in traffic congestion.
 18. The CDMA subscriber unitof claim 10 further comprising a display configured to display adestination address.
 19. A code division multiple access (CDMA)subscriber unit comprising: an antenna configured to receive a firstsignal from a first base station and a second signal from a second basestation, wherein the first signal is spread coded in accordance with afirst pseudorandom code and the second signal being spread coded inaccordance with a second pseudorandom code; and a circuit operativelycoupled to the receiver configured to measure a difference, in chips,between a first arrival time of a portion of the first pseudorandom codecarried by first signal and a second arrival time of a portion of thesecond pseudorandom code carried by the second signal; wherein thecircuit is further configured to transmit a spread coded third signalcarrying information indicative of the difference between the first andsecond arrival times to a base station associated with one of the firstand second base station using the antenna.
 20. The CDMA subscriber unitof claim 19 further comprising a display configured to display alocation of the subscriber unit, wherein the location is based on thedifference between the first and second times of arrival.
 21. The CDMAsubscriber unit of claim 19 further comprising a speaker configured togenerate an audible signal, wherein the audible signal indicates alocation of the subscriber unit that is based on the difference betweenthe first and second times of arrival.
 22. The CDMA subscriber unit ofclaim 19 further comprising a display configured to display instructionsfor reaching a destination address.
 23. The CDMA subscriber unit ofclaim 22, wherein the display is configured to modify the instructionsin response to changes in traffic congestion.
 24. The CDMA subscriberunit of claim 19 further comprising a display configured to display adestination address.
 25. The CDMA subscriber unit of claim 19, whereinthe first and second signals carry data.